Diester/ether diols of tetrabromophthalic anhydride are well known reactive flame retardants. See for example, U.S. Pat. No. 4,564,697 to B. J. Sutker. Such products can have bromine contents of 40 wt % or more. However they are viscous liquids with viscosities in the 80,000 to 200,000 cps range at 25° C., with 100,000 cps being typical. In order to pump such liquid products it is necessary to heat them to elevated temperatures. For example one commercial product with a viscosity at 25° C. in the range of 80,000 to 135,000 cps when heated to 60° C. will typically have a viscosity in the range of 1400 to 2100. To avoid the need for heating the product to reduce its viscosity, a commercially-viable reactive flame retardant product has been produced as a blend of diester/ether diol of tetrabromophthalic anhydride, polyol, and a liquid phosphate ester. Although this product has typical viscosities in the 6000 to 10,000 cps range at 25° C., its bromine content is reduced to a typical value of 36 wt %.
A need thus exists for polyols based on tetrabromophthalic anhydride which are effective as flame retardants, which have low viscosities at 25° C. (e.g., about 20,000 cps or less), which have high halogen contents (e.g., at least about 40 wt %), and which can be produced economically.